This invention relates to electrotherapy circuits and in particular to a defibrillator having an improved self test operation that requires less energy.
Electro-chemical activity within a human heart normally causes the heart muscle fibers to contract and relax in a synchronized manner that results in the effective pumping of blood from the ventricles to the body's vital organs. Sudden cardiac death is often caused by ventricular fibrillation (VF) in which abnormal electrical activity within the heart causes the individual muscle fibers to contract in an unsynchronized and chaotic way. The only effective treatment for VF is electrical defibrillation in which an electrical shock is applied to the heart to allow the heart's electro-chemical system to re-synchronize itself. Once organized electrical activity is restored, synchronized muscle contractions usually follow, leading to the restoration of cardiac rhythm.
The necessity to apply defibrillation quickly after onset of VF has given rise to automatic external defibrillators (AEDs) which may be used by first responders and lay people. AEDs may remain unused for long periods of time and yet must be ready to operate reliably in an emergency situation. To ensure operational readiness, most AEDs employ a self test operation that is conducted at regular intervals.
The Heartstream Forerunner.RTM. AED, for example, employs a self test system that generates self test operations automatically in response to a predetermined schedule. The self test operation typically includes a number of different system checks including functional, calibration, and safety tests to verify that the defibrillator's components and operation are within predetermined specifications. The high voltage (HV) circuit is a critical component of the defibrillator that provides the defibrillation pulse. Verification of the proper functioning of the defibrillator is a typical part of any self test operation.
U.S. Pat. No. 5,591,213, "Defibrillator System Condition Indicator", issued Jan. 7, 1997, to Morgan et al., describes a defibrillator system which includes means for periodically operating a high voltage (HV) circuit to discharge a test pulse to a test load. Such self tests may be done periodically or in response to changes in the defibrillator environment such as the ambient temperature as described in U.S. Pat. No. 5,868,792, issued Feb. 9, 1999, "Environment-Response Method for Maintaining Electronic Devices Such As An External Defibrillator", issued Feb. 9, 1999, to Ochs et al. which is incorporated herein by reference.
U.S. Pat. No. 5,800,460, "Method for Performing Self-Test In A Defibrillator", issued Sep. 1, 1998, to Powers et al., describes in detail the operation of a defibrillator self test system which is incorporated herein by reference. An energy storage capacitor is twice charged to full voltage and discharged, first to functionally verify operation of the HV circuit under combined maximum voltage and current conditions and second to calibrate the HV circuit to ensure that the amount of energy delivered in the defibrillation pulse is within specification limits. The test load is resistance typically in the range of 10 to 20 ohms.
Providing a test pulse to the test load at a combined maximum voltage and maximum current stress as taught in by Powers et al. results in a substantial amount of energy dissipated in the test load for each self test operation. It has been found that over time the periodic self test operations conducted by the AED substantially reduced its battery life. The self test of the HV circuit forms only a portion of the overall defibrillator self test but consumes the majority of total energy that is required.
U.S. Pat. No. 5,873,893, "Method and Apparatus for Verifying the Integrity of an Output Circuit Before and During Application of a Defibrillation Pulse", to Sullivan et al. describes an external defibrillator capable of testing the high voltage output circuit for open and shorted switches by monitoring the energy storage capacitor voltage. The high voltage circuit, the well known H-bridge configuration, is tested first by sequentially turning on each of the switches in each leg of the H-bridge while the energy storage capacitor is charged up to a test voltage. No current conducting paths should appear through the H bridge. Next, current conducting paths through each side of the H bridge are created for a brief time and the energy storage capacitor voltage is monitored. The energy storage capacitor voltage should drop by a predetermined amount for each discharge. While it is mentioned by Sullivan et al. that the test voltage may be less than the maximum voltage to reduce energy consumption, there is no teaching on how the test voltage may differ between the open and short circuit tests to allow for full testing of HV circuit at both maximum current and maximum voltage levels.
It would therefore be desirable to provide a method and apparatus of reduced energy self test in a defibrillator which allows for testing of the HV circuit at both maximum current and maximum voltage.